Following the lead of Neil DeGrasse Tyson on Twitter, I thought I’d write a quick note in honor of St. Patrick’s Day (at least as those of us in the United States tend to stereotype it) and the associated color green. I have taught astronomy quite a few times over the years (though I’m not technically an astronomer), and one weird fact that always boggles students minds is that although there are red stars, orange stars, yellow stars, white stars, and blue stars, there are no green stars (or purple stars, for that matter).
In fact, our Sun’s light peaks in the green part of the spectrum. The figure (click to enlarge) shows the energy output of stars with a variety of surface temperatures, known as thermal spectra. Our Sun has a surface temperature in between the yellow and green lines, so it peaks in the middle of the visible light part of the spectrum (roughly 400 to 700 nanometers (nm), or 0.000016 inches to 0.000028 inches for those who aren’t metrical)—right around the green part of the rainbow. But here’s the deal: if you look at that green line, you see that it may peak at green, it spreads out and covers most of the rainbow, so we actually would see that “green” star—our Sun*!—as white, which is a mixture of all the colors!
In addition, human are optimized for the light our Sun produces, so our receptor cells have peak performance right at the maximum solar output. (Other animals are able to see infrared and/or ultraviolet light, and have different levels of color perception.) Thus, “green” stars will never appear as a green color to human eyes simply because we are adapted to life under our Sun—any similar star or object with the same temperature range will also be perceived as white.
* As a side note, we often refer to our Sun as yellow, but that’s also a bit of a misnomer: if our Sun were actually yellow, white objects (which reflect back all the visible colors equally) would appear yellow as well.
Galileo's Pendulum 
8 responses to “It’s Not Easy Being Green”
[…] best way to see the hot gas directly is through the light it emits. In contrast to thermal emission like what we see from our Sun, the gas between galaxies in a cluster produces something known as breaking radiation or (if […]
[…] a very important role in the history of physics. Specifically, Max Planck found he could explain thermal radiation by assuming light comes in bundles of energy we now call photons; Albert Einstein extended this idea […]
[…] As a result, an interesting pattern of light emerges, known as the thermal spectrum. As I wrote in an earlier post about why there are no green stars, hotter objects emit more light than cooler objects, again because there is more energy available […]
[…] which is a way to remember the spectral classification of stars. Here’s how it works: the color of a star is determined by its temperature, as I outlined […]
[…] That’s what our first census will see inside the box: galaxies, whose light is from stars. Human eyes are tuned to a narrow part of the whole spectrum of light, which is ideal for seeing thos…. Changing the type of light would show a different view: infrared will still show the same […]
[…] I mentioned in earlier posts on the subject, the color of a star is dictated by its surface temperature. Hot stars appear blue or blue-white, while cool stars […]
[…] reason no star appears green, then, isn’t because they don’t emit green light, but because register light across the entire visible light spectrum. If we could imagine a […]
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